if youve got an appropriately specced DVR card with decent software then it does it all for you. most good software will allow you to choose both the quality of the recording, frame rate, with/without sound etc etc etc

Also you can choose how much of the hard drive you use for recording. You can set up folders etc to keep recordings long term or have a rolling system where when its hard drive partition is full, it starts to overwrite the oldest material.

What i finally want for mine is like a 4 camera system including a PTZ dome all recorded in the same quality 24/7. would need a big hard drive though!!